This triggers an assert() failure on debug-mode Python (or a leak in release Python):

from itertools import groupby

def f(n):
    print("enter:", n)
    if n == 5:
        list(b)
    print("leave:", n)
    return n != 6

for (k, b) in groupby(range(10), f):
    print(list(b))

With current trunk we get: python: ./Modules/itertoolsmodule.c:303: _grouper_next: Assertion `gbo->currkey == NULL' failed.

Do you have a suggested fix?

PR 1557 implements the simplest fix -- remove the assert and use Py_XSETREF() instead of simple assignment. Since the resulting code in _grouper_next() is identical to the code in groupby_next(), it was shared between these two functions.

At first glance, the suggested PR looks fine.  Unless there is a rush, I would like to hold-off spend more time thinking about Issue 30346 before moving forward with this one.

Hmm, the sole issue30346 is not enough for fixing this crash. I don't know if there is a simpler fix based on issue30346, but since PR 1557 removes the code duplication I'm going to merge it in any case.

New changeset c740e4fe8a9bc5815dc18c38d7f7600b128c3c51 by Serhiy Storchaka in branch 'master':
bpo-30347: Stop crashes when concurrently iterate over itertools.groupby() iterators. (#1557)
https://github.com/python/cpython/commit/c740e4fe8a9bc5815dc18c38d7f7600b128c3c51

New changeset 69b2dc8637ba924d78f9869be592c5a545510f10 by Serhiy Storchaka (Miss Islington (bot)) in branch '3.6':
[3.6] bpo-30347: Stop crashes when concurrently iterate over itertools.groupby() iterators. (GH-1557) (#3770)
https://github.com/python/cpython/commit/69b2dc8637ba924d78f9869be592c5a545510f10

New changeset d0b9dc33676bdad217d5074954c1b37d4ca54a10 by Serhiy Storchaka in branch '2.7':
[2.7] bpo-30347: Stop crashes when concurrently iterate over itertools.groupby() iterators. (GH-1557). (#3772)
https://github.com/python/cpython/commit/d0b9dc33676bdad217d5074954c1b37d4ca54a10